1. Field of the Invention
The present invention relates to an exposure apparatus and an exposure method which can be applied, for example, to an exposure apparatus for a disc master to be used to form an optical disc. An SHG exposure laser beam having a wavelength of 300 nm or less is modulated by a modulation means, and applied to a disc master by proximity effect using an objective lens of a numerical aperture of 1.0 or more, whereby an optical disc having a substantially higher recording density as compared to that of conventional discs is obtained. According to the present invention, it is possible to perform exposure on the disc master of such an optical disc.
2. Description of the Related Art
Conventionally, in the production of an optical disc, exposure is performed on a disc master by an exposure apparatus, and then the disc master is developed to prepare a mother disc, and a stamper is prepared from the mother disc for mass production of optical discs.
FIG. 6 is a plan view of an exposure apparatus of this type seen from above. In this exposure apparatus 1, exposure is effected on a disc master 2 by an exposure laser beam, whereby a latent image corresponding to the pits and grooves is formed on the disc master 2.
To prepare the disc master 2, precision polishing is performed on a glass disc having a diameter of approximately 200 mm and a thickness of several mm, and a resist layer having a thickness of approximately 0.1 xcexcm is formed thereon by spin coating of photoresist. As the photoresist, a photosensitive material exhibiting a sufficient sensitivity to the exposure laser beam is applied. The disc master 2 is attached to an air spindle through chucking and held by this exposure apparatus 1 to be rotated at a predetermined speed.
As the laser light source 3, a Kr ion laser, for example, is used, which emits a laser beam having a wavelength of 413 nm as an exposure laser beam L1. Mirrors 4 and 5 bends the optical path of the exposure laser beam L1 emitted from the laser light source 3 and leads it to an EOM (electro optic modulator) 6. The EOM 6 rotates the plane of polarization of the exposure laser beam L1 in response to a drive signal and emits the beam. Subsequently, a polarization beam splitter 7 selectively allows a predetermined polarization plane component of the exposure laser beam L1 to be transmitted.
A half mirror 8 divides the exposure laser beam L1 emitted from the polarization beam splitter 7 into two beams, and a photoreceptor 9 receives the beam transmitted through the half mirror 8 and outputs the light quantity detection result. In the exposure apparatus 1, the drive signal of the EOM 6 is corrected based on the light quantity detection result to thereby form an automatic light quantity control circuit, effecting control such that the light quantity of the exposure laser beam L1 is kept constant.
A lens 10 condenses the exposure laser beam L1 reflected by the half mirror 8 and inputs it to an AOM (acousto optic modulator) 11, which ON/OFF-modulates the exposure laser beam L1 by a modulation signal corresponding to a pit row. Subsequently, a lens 12 converts the beam output from the acousto optic modulator 11 to a parallel beam and outputs it. A half mirror 13 divides the beam output from the lens 12 into two beams. A photoreceptor 14 receives one of the two beams and outputs the reception result, whereby in the exposure apparatus 1, the result of modulation of the exposure laser beam L1 by the acousto optic modulator 11 can be monitored.
On the other hand, a concave lens 15 outputs the other one of the two beams obtained through division by the half mirror 13 as a diverging ray. Subsequently, a convex lens 16 converts the diverging ray to a parallel beam. Thus, the concave lens 15 and the convex lens 16 constitute a beam expander, and outputs the exposure laser beam L1 after setting the beam diameter to a predetermined value.
A mirror 17 receives the exposure laser beam L1 from the beam expander through beam splitter 18, and emits this exposure laser beam L1 toward the disc master 2. An objective lens 19 is formed by a lens similar to the objective lens of a microscope. As shown in FIG. 7, the exposure laser beam L1 whose optical path has been bent by the mirror 17 is condensed on the resist layer of the disc master 2 to thereby form a pit latent image.
When thus performing exposure on the disc master 2 by the exposure apparatus 1, the exposure laser beam L1 is reflected by the resist layer of the disc master 2, and the resulting return beam L2 travels in the opposite direction through the optical path of the exposure laser beam L1 to impinge upon the half mirror 13. Mirrors 22, 23 and 24 sequentially bend the optical path of the return beam transmitted through the half mirror 13, and a lens 25 guides the return beam reflected by the mirror 23 to an imaging device 26 consisting of a CCD camera. The imaging device 26 receives this return light to thereby detect the beam configuration of the exposure laser beam L1 on the resist layer of the disc master 2. Due to this arrangement, the exposure apparatus 1 can monitor to check as to whether the focus control is correctly effected or not through the observation of the beam configuration. Further, it is possible to set the control target in the focus control.
In the exposure apparatus 1, the optical system from the laser light source 3 to the half mirror 13, which processes the exposure laser beam L1, and the optical system from the half mirror 13 to the imaging device 26, which receives the return beam, are secured to an optical base plate which is the base of this exposure apparatus. In contrast, the optical system from the concave lens 15 to the objective lens 19 is arranged on a movable optical table 29, which can move in the radial direction of the disc master 2 by means of a predetermined drive mechanism. Due to this arrangement, in the exposure apparatus 1, the movable optical table 29 is gradually moved in the peripheral direction of the disc master 2, with the disc master 2 rotating, whereby the scanning trail of the exposure laser beam is spirally formed on the disc master 2, and a pit row latent image according to the modulation by the acousto optic modulator 11 is formed on this scanning track.
In the exposure apparatus 1, there is further formed an auto focus optical system on the movable optical table 29. In the auto focus optical system, a laser diode 30 emits a laser beam LF having a wavelength of, for example, 680 nm, and a polarization beam splitter 31 reflects this laser beam LF and emits it to a dichroic mirror 17. A xc2xc wavelength plate 32 imparts a phase difference to the laser beam LF emitted from the polarization beam splitter 31 before emitting it, and a beam splitter 18 synthesizes the laser beam LF with the exposure laser beam LR before emitting it to the mirror 17. Thus, in the auto focus optical system, the laser beam LF is applied to the disc master 2 together with the exposure laser beam LR.
The laser beam LF, which has a beam diameter much smaller than that of the exposure laser beam LR, is synthesized with the exposure laser beam LR. Further, the synthesis is effected such that the optical axis of the laser beam LF is spaced apart from the optical axis of the exposure laser beam LR, the optical axis of which substantially coincides with the optical axis of the optical system including the objective lens 19, etc.
Due to this arrangement, in the auto focus optical system, regarding the return beam, which is obtained through specular reflection at the resist layer of the disc master 2 of the laser beam LF obliquely impinging upon the disc master 2, the position of the optical axis varies according to the distance between the objective lens 19 and the resist layer. Regarding the return beam thus obtained, the auto focus optical system imparts a phase difference thereto when it reversely travels through the optical path of the laser beam LF and is transmitted through the xc2xc wavelength plate 32, whereby the laser beam LF is subsequently separated by the polarization beam splitter 32. The beam is further received by a position detecting device 33, and the distance between the objective lens 19 and the resist layer is detected from the light receiving position.
In the auto focus optical system, the optical axis position, etc. of the laser beam LF is adjusted such that the variation in the condensing position of the return beam at the position detecting device is approximately 100 times the variation in the distance between the objective lens 19 and the resist layer, and the objective lens 19 is displaced in the optical axis direction to thereby effect focus control.
The exposure apparatus 1 is mounted on an air base plate so that the optical systems and the mechanical system may not be affected by external vibrations of the place of installment, whereby the exposure accuracy can be improved.
In the exposure apparatus 1, which performs exposure on the disc master 2 in this way, assuming that the resolution is P, P=Kxc2x7(xcex/NA), where NA is the numerical aperture of the objective lens 19, K is the process factor (usually 0.8 to 0.9) due to resist characteristics, etc., and xcex is the wavelength of the exposure laser beam L1. Thus, for example, in a DVD having a diameter of 12 cm and an information capacity on one side of 4.7 GB, a pit row latent image is prepared in a minimum pit length of 0.4 xcexcm and a track pitch of 0.74 xcexcm to secure a resolution P=0.37 xcexcm by using an exposure laser beam having a wavelength of 413 nm and an objective lens 19 of a numerical aperture NA=0.9.
Regarding optical discs, with the recent rapid development in information/communication techniques and image processing techniques, there is a demand for an increase in capacity.
When an information capacity of 15 GB on one side is to be secured using, for example, a disc having the same diameter as DVD (12 cm) and in the same format as DVD, this capacity can be secured forming a pit row in a minimum pit length of 0.22 xcexcm and a track pitch of 0.41 xcexcm.
In this case, in the exposure apparatus, it is necessary to secure a resolution corresponding to the minimum pit length from the above formula of the resolution P. Regarding the numerical aperture NA, the present NA=0.9 is to be considered as the limit from the accuracy in lens design. Thus, by reducing the wavelength of the exposure laser beam and using a far-ultraviolet radiation laser having a wavelength of approximately 250 nm, it is possible to secure the capacity of 15 GB.
However, to further increase the recording capacity and secure an information capacity on one side of, for example, 40 GB in the diameter of 12 cm, it is not enough to simply reduce the wavelength of the exposure laser beam L1.
That is, to secure an information capacity on one side of 40 GB in the diameter of 12 cm, assuming that recording is to be performed in the same format as DVD, it is necessary to form a pit row in a minimum pit length of 0.14 xcexcm and a track pitch of 0.25 xcexcm.
The present invention has been made in view of the above problem. Accordingly, it is an object of the present invention to provide an exposure apparatus and an exposure method for an optical disc in which recording density has been substantially increased.
In an exemplary embodiment, an SHG exposure laser beam having a wavelength of 300 nm or less is used, so that it is possible to emit a laser beam having a short wavelength and suitable for exposure, using a small size light source. Further, by applying an exposure laser beam to a disc master by near field effect to a disc master by using an objective lens of a numerical aperture of 1.0 or more, it is possible to form a much more minute latent image, and perform exposure on a disc master for an optical disc in which the recording density has been substantially increased.